Frequency synthesizers may be employed in a variety of applications, including radio receivers, radio telephones, mobile telephones, portable telephones, walkie-talkies, CB radios, satellite receivers, GPS systems, navigation systems, wireless networking systems and the like. Frequency synthesizers may be employed to provide an output signal with a variable frequency dependent upon a frequency of an input signal. Performance of many devices which employ frequency synthesizers, such as receivers, may be dependent upon the performance of the frequency synthesizer. For example, frequency synthesizers which operate quickly and create minimal phase noise (rapid, short-term random fluctuations in the phase of a wave, caused by time domain instabilities) perform optimally.
Analog frequency synthesizers may be direct or indirect. Indirect analog frequency synthesizers use analog voltage controlled oscillators as the primary frequency generating device. By comparing the output frequency of the voltage controlled oscillator with a stable reference an error signal is produced to precisely control the output frequency. Direct analog frequency synthesizers utilize multiple RF techniques to translate and multiply reference frequencies to a desired frequency range. Essentially, an output is selected by means of high-speed switching among different constantly generated frequencies.
Conventional applications of ultra low phase noise indirect analog synthesizers employ a plurality of loops to keep the multiplier value (N) low. Conventional synthesizers may incorporate step recovery diodes or sampling detectors to create a low noise source to inject in an offset loop.
However, step recovery diodes and sampling detectors are not ideal multipliers. Additionally, loops containing step recovery diodes and sampling detectors have to be steered to the right comb (a set of frequency harmonics) so that the loop locks to the proper frequency. This adds complexity to the synthesizer. Additionally, multiple loops add to the latency of the synthesizer since the main loop must wait for settling of the other loops before the main loop may settle on a frequency. Consequently, replacing step recovery diodes or sampling detectors with simpler and faster components having better phase noise performance would be desirable.